1. Field of the Invention
This invention relates to a catalyst for reducing nitrogen oxides in exhaust gases containing ammonia, and particularly it relates to a catalyst for catalytic reduction, wherein the activity of the catalyst is resistant to deterioration by volatile catalyst poisons such as arsenic (As), selenium (Se), tellurium (Te), etc. contained in exhaust gases.
2. Description of the Related Art
Nitrogen oxides contained in exhaust gases from various combustion furnaces are not only harmful by themselves to the human body, but are also causes of air pollution such as photochemical smog. In order to remove such nitrogen oxides or denitrate, a catalytic reduction process with ammonia (selective reduction) has now been broadly employed. As to the catalyst utilized in the reduction process, a number of catalysts have so far been invented, and among these catalysts, those which have been practically used are catalysts composed mainly of titania (titanium dioxide) to which are added vanadium (V), molybdenum (Mo), tungsten (W) or the like. They are representatively disclosed in Japanese patent application laid-open Nos. Sho 50-51966/1975 and Sho 52-122293/1977. These catalysts are superior in that activity deterioration due to sulfur oxides, etc. are small. However, they have a drawback in that resistance to volatile catalyst poisons such as As, Se, Te, etc. is so low that activity deterioration is notable.
On the other hand, in recent years, coal and crude oil from China containing a large amount of mineral substances have been used so that there is a tendency that the concentration of the above-mentioned metals in exhaust gases increases. In such a situation, when the above-mentioned catalysts are used for denitrating such exhaust gases, a new problem has developed concerning the lowering of the catalyst's activity.